在Java中，要實現MQTT消息的發布和訂閱，你需要使用一個支持MQTT協議的庫。Eclipse Paho是一個流行的MQTT客戶端庫，提供了Java版本。以下是使用Eclipse Paho庫進行MQTT消息發布和訂閱的步驟：

1. 首先，將Eclipse Paho庫添加到你的項目中。如果你使用Maven，可以在pom.xml文件中添加以下依賴：

   <groupId>org.eclipse.paho</groupId>
   <artifactId>org.eclipse.paho.client.mqttv3</artifactId>
   <version>1.2.5</version>
</dependency>

2. 創建一個MQTT客戶端并連接到MQTT代理服務器（Broker）：

import org.eclipse.paho.client.mqttv3.IMqttClient;
import org.eclipse.paho.client.mqttv3.MqttClient;
import org.eclipse.paho.client.mqttv3.MqttConnectOptions;
import org.eclipse.paho.client.mqttv3.MqttException;

public class MqttExample {
    public static void main(String[] args) {
        String broker = "tcp://localhost:1883";
        String clientId = "JavaSampleClient";
        
        try {
            IMqttClient mqttClient = new MqttClient(broker, clientId);
            MqttConnectOptions options = new MqttConnectOptions();
            options.setCleanSession(true);
            mqttClient.connect(options);
            System.out.println("Connected to MQTT broker");
        } catch (MqttException e) {
            e.printStackTrace();
        }
    }
}

3. 訂閱主題（Topic）以接收消息：

import org.eclipse.paho.client.mqttv3.IMqttDeliveryToken;
import org.eclipse.paho.client.mqttv3.MqttCallback;
import org.eclipse.paho.client.mqttv3.MqttMessage;

// ...

public class MqttExample {
    // ...
    
    private static void subscribe(IMqttClient mqttClient) {
        try {
            mqttClient.subscribe("my/topic", 0, new MqttCallback() {
                @Override
                public void connectionLost(Throwable cause) {
                    System.out.println("Connection lost");
                }

                @Override
                public void messageArrived(String topic, MqttMessage message) throws Exception {
                    System.out.println("Received message on topic: " + topic);
                    System.out.println("Message content: " + new String(message.getPayload()));
                }

                @Override
                public void deliveryComplete(IMqttDeliveryToken token) {
                    System.out.println("Message delivered");
                }
            });
            System.out.println("Subscribed to topic: my/topic");
        } catch (MqttException e) {
            e.printStackTrace();
        }
    }
}

4. 發布消息到主題：

private static void publish(IMqttClient mqttClient) {
    try {
        String topic = "my/topic";
        String content = "Hello, MQTT!";
        int qos = 0;
        boolean retained = false;
        
        MqttMessage message = new MqttMessage(content.getBytes());
        message.setQos(qos);
        message.setRetained(retained);
        
        mqttClient.publish(topic, message);
        System.out.println("Message published to topic: " + topic);
    } catch (MqttException e) {
        e.printStackTrace();
    }
}

5. 在main方法中調用這些方法：

public static void main(String[] args) {
    // ...
    
    try {
        IMqttClient mqttClient = new MqttClient(broker, clientId);
        MqttConnectOptions options = new MqttConnectOptions();
        options.setCleanSession(true);
        mqttClient.connect(options);
        System.out.println("Connected to MQTT broker");
        
        subscribe(mqttClient);
        publish(mqttClient);
        
        // Keep the client running for a while to receive messages
        Thread.sleep(60000);
        
        mqttClient.disconnect();
        System.out.println("Disconnected from MQTT broker");
    } catch (MqttException | InterruptedException e) {
        e.printStackTrace();
    }
}

這個示例展示了如何使用Eclipse Paho庫在Java中實現MQTT消息的發布和訂閱。你可以根據自己的需求修改代碼，例如更改主題、消息內容或質量等級（QoS）。